Interaction between K adatoms on graphene is investigated by first-principles calculations based on density function theory and analytical analyses based on the k . p perturbation theory. The calculation shows that there is a strong repulsion between K adatoms. The main origin of this strong repulsion is not from the dipole-dipole interaction as suggested for K adatoms on graphite surface, but comes from the screened Coulomb interaction. Potassium adatom on graphene donates its s electron and becomes K+. The positively charged K adatom induces electron density oscillation on graphene which is responsible for the screened Coulomb repulsion between the K adatoms.